Jul 302014

Have you heard of ventilator-associated events (VAEs)? Like it or not, this neologism of healthcare-speak is coming to an ICU near you soon. Here's the lowdown on VAEs and why they matter to the practicing intensivist.

What are Ventilator-Associated Events?

Ventilator-associated events are an invention of the Centers for Disease Control and Prevention (CDC), created in response to the difficulty in getting good epidemiologic data for ventilator-associated pneumonia (VAP). Reliable public health data on ventilator associated pneumonia are hard to collect for at least three reasons:

  1. The requirement for radiographic pneumonia to make a VAP diagnosis results in many missed cases (i.e. chest films are not sensitive for ventilator-associated infections).
  2. In response to the threat of nonpayment for ventilator associated pneumonias, or a loss in their quality rankings, hospitals have reduced their VAP case rates to zero (or nearly so) -- although few observers believe that VAPs have in reality been eradicated.
  3. Non-infectious conditions (e.g., pulmonary edema, ARDS) can easily be misdiagnosed as VAP, and vice versa.

Because virtually all serious respiratory conditions in mechanically ventilated patients result in an increasing requirement for oxygen, the CDC used an increase in need for oxygen or positive end-expiratory pressure (PEEP), after a previous stable baseline of at least 2 days, as the case definition of a ventilator-associated event or VAE.

By substituting the oxygenation criteria for the VAP radiographic criterion, CDC hopes to improve its data collection on VAPs, and in the process also gather information on non-infectious conditions in mechanically ventilated patients.

What are the types of Ventilator-Associated Events?

The CDC has erected a hierarchy of ventilator-associated events: ventilator-associated conditions (VAC), infection-related ventilator-associated complications (IVAC), and ventilator-associated pneumonia (VAP), which may be possible or probable:

  • Any increase of oxygen requirement ( ≥0.20 in FiO2) or PEEP (≥3 cm H2O) after a period of stability (≥2 days) without evidence of infection is a ventilator associated condition (VAC).
  • If temperature is >38° or <36°, or white blood cell count > 12,000/mm³ or <4,000/mm³, AND a new antibiotic is added for at least 4 days, along with the oxygenation change, that's an infection-related ventilator associated complication (IVAC).
  • If there are also purulent secretions or positive respiratory cultures (irrespective of chest film findings), that's a ventilator associated pneumonia (VAP), possible or probable, influenced by numerous criteria.

There are many additional nuances omitted here for concision. CDC has created a VAE calculator for its authorized agents to standardize their public health surveillance for VAEs.

Why should you care about VAEs?

The CDC explicitly says in its VAE guide that "the VAE definition algorithm is for use in surveillance; it is not a clinical definition algorithm and is not intended for use in the clinical management of patients."

But that's a bit disingenuous. Practically speaking, hospitals know that they will be measured on ventilator-associated event rates, possibly with future financial or public relations consequences. Many hospitals have already begun using the CDC's VAE criteria to identify VAEs in real time inside their ICUs. You may be approached by your hospital's infection control or quality improvement teams after they've identified a ventilator-associated event in one of your patients.

What should you expect from the use of VAEs?

Conceivably, VAEs could be flagged by trained observers at your hospital (respiratory therapy supervisors, nursing leadership, etc) using the CDC calculator and brought up for discussion on interdisciplinary rounds each day alongside the growing panoply of surveilled parameters (DVT prophylaxis, sepsis screening, et al). At that point, significant social / professional pressure will exist for the physician to "do something," moreso than under the current status quo.

Ventilator-associated events are a formal establishment of an already-enshrined concept in the treatment of critically ill mechanically ventilated patients, which is to take changes in oxygen and/or PEEP requirements seriously and to seek an underlying treatable cause.

The challenge, of course, is acting only on the meaningful changes in oxygenation/PEEP that reflect a true deterioration. If "doing something" treats a real clinical problem (e.g. reducing tidal volumes for a patient entering ARDS), that could be good. If it means giving antibiotics to tens of thousands of people with atelectasis and small pleural effusions across the U.S. yearly, that's not so great.

Does the VAE concept really work in practice?

The best evidence suggests that the VAE concept is not useful for guiding clinical decisions in the moment. Its performance characteristics as a screening test appear to be terrible, with poor sensitivity (~32%) for detecting VAP in the one of the only prospective studies. This is because (as will not surprise the practicing intensivist) clinically insignificant fluctuations in oxygenation/PEEP status are often recorded as VAEs, diluting signal with noise.

Numerous retrospective reviews supporting the VAE concept listed on CDC's website strongly link VAEs with morbidity and mortality. However, that's very different from showing that a prospective (intervention-based) approach is helpful, over and above standard care (VAP and DVT prevention, low tidal volumes, fluid restriction, etc).

Pneumonia expert Dr. Richard Wunderink has said "the central hypothesis of the VAE criteria—that VAP and other potentially preventable complications of mechanical ventilation can consistently be detected by worsening gas exchange—is clearly not true."

What should you do about ventilator-associated events?

Separating trivial day-to-day fluctuations in oxygenation (such as from atelectasis due to repositioning) from true deterioration is a central challenge in the care for mechanically ventilated patients.

In any patient requiring an increase in oxygen concentration (FiO2) or PEEP after a period of relative stability, always consider the main treatable causes:

  • Pulmonary edema, usually from iatrogenic fluid overload
  • Ventilator-associated pneumonia (VAP)
  • Acute respiratory distress syndrome (ARDS)
  • Pulmonary embolism
  • Pneumothorax
  • Sepsis
  • Atelectasis

Checking a chest film (or ultrasound if you are skilled at it), arterial blood gas, respiratory cultures (tracheal aspirate, mini-BAL or bronchoscopic BAL), reducing tidal volumes to 6 ml/kg ideal body weight, providing diuretics, selective use of CT-angiograms, and empiric antibiotics are the most frequent interventions (among others), that must be individualized for each patient with a meaningful deterioration in oxygenation status.

The best outcome from the impending promotion of the VAE concept would be more awareness and earlier action among physicians in identifying and treating real clinical deteriorations (as opposed to blindly following an algorithm).

Ventilator-Associated Events: Clinical Takeaway

Ventilator-associated events are changes in oxygen requirement after a period of stability, being used by the CDC to try to better monitor the epidemiology of ventilator-associated pneumonia. VAEs are not well validated, and have not been sensitive for identifying VAP cases in the few prospective data available.

More concerningly, the VAE concept is being repurposed by many hospitals to try to identify patients' deteriorations at an earlier, treatable stage -- a use of VAEs that (to my knowledge) has not been validated at all. This well-meaning effort could have both positive and negative effects, but will certainly generate many clinically insignificant VAEs requiring consideration.

It's at least possible that the heightened attention paid to patients' status using VAEs could drive culture change, improving overall care and adherence with things like low tidal volumes and fluid restriction for ARDS, which work but aren't widely practiced today. There's a precedent here in sepsis protocols, which (although apparently unnecessary for great care) likely helped improve overall care of sepsis worldwide, as have sedation protocols for ventilator care.

But those changes were sparked by initial randomized trials showing benefit: a critical difference. Ventilator-associated events are an invention, an arbitrary definition looking for a reality in which they could be relevant or useful. So far, the only place they meaningfully exist seems to be spreadsheets and clipboards -- not in the real world of data-driven clinical care.

More from the CDC:

Ventilator-associated events surveillance home page

Guide for CDC's VAE monitors (PDF)

CDC's ventilator-associated events calculator

Journal articles on the ventilator associated event concept

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What are Ventilator-Associated Events (and why should you care)?